Knowing about religion is crucial, because "religion is the most powerful piece of culture," Prothero said in a phone interview from his home in Boston. It fuels wars and inspires justice, and "after 9/11 it's really hard to pretend anymore that religion doesn't matter. That it's not literally a life-and-death matter.

"What would have happened," he asks, "if George Bush and Dick Cheney and Condoleezza Rice were religiously literate?" What would have happened if the three of them, and others in the Departments of State and Defense, had understood the religious complexity of Iraq. "At least they would have said, 'OK, we have Sunnis and Shiites, and maybe we're going to get a civil war."'

Prothero is equally dismayed by Texas Rep. Silvestre Reyes, Democratic chairman of the House Intelligence Committee, who was asked several weeks ago by a reporter whether al-Qaida is a Sunni or Shiite organization. Predominantly Shiite, Reyes answered.

"To me, that's unforgiveable," Prothero says. "That's like someone running the Department of Transportation and not knowing the difference between a highway and a country lane."

Domestically, Prothero argues, Americans need religious literacy in order to be "effective citizens." Literacy about Christianity is especially important, he says, because Christian references are used in political arguments about issues such as abortion, stem cell research and capital punishment.

When people argue that abortion is wrong because the Bible says so, "we should be able to say, where does the Bible say that, and the person should be forced to come back and show us," he says. "Or, on the left, we may hear that capital punishment is wrong 'because the Bible says so.'

"The point is that when people on the right or left make this kind of talk, they draw down the authority of God or religion or Christ or the Bible to support their views, and sometimes it's totally bogus and sometimes it's not. You can engage them by saying 'don't refer to religion.' That's the civil libertarian approach. But that's not going to change it."

The decline in religious studies in America was not a result of the 1962 Supreme Court ruling outlawing prayer in schools, he argues. Instead, there were a series of cultural shifts beginning in the 1800s. "Ironically," he writes, "the United States became a nation of forgetters at the same time it became a nation of evangelicals." Believing in Christ became more important, he writes, than knowing about Christ. "To evangelicalism, therefore, we owe both the vitality of religion in contemporary America and our impoverished understanding of it."